This application is based on patent application No. 2000-056377 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a discharge tube for a local etching apparatus and a local etching apparatus using the discharge tube.
2. Description of the Related Arts
FIG. 8 is a schematic sectional view showing an example of a conventional local etching apparatus. This local etching apparatus has a discharge tube 100, a gas supply unit 110, a microwave oscillator 121, and a stage 130.
According to this construction, gas is fed from the gas supply unit 110 to the discharge tube 100, while from the microwave oscillator 121 is generated a microwave M into a waveguide 122 to convert the gas present in the discharge tube 100 into plasma and an active species G contained in the plasma is jetted from an orifice 101 of the discharge tube 100 onto a silicon wafer W placed on the stage 130.
Then, by moving the stage 130 horizontally, a surface portion (hereinafter referred to as the xe2x80x9crelatively thick portionxe2x80x9d) Wa which is thicker than a specified surface thickness of the silicon wafer W is conducted to a position just under the orifice 101 of the discharge tube 100 and the active species G is jetted to the relatively thick portion Wa from the orifice 101 to etch the relatively thick portion Wa locally. The entire surface of the silicon wafer W is thus etched locally, whereby the surface thickness distribution of the silicon wafer W is rendered uniform and the entire surface of the wafer is made flat.
The conventional discharge tube 100 used in the above local etching apparatus has such a cylindrical structure: as shown in FIG. 9, which has a constant wall thickness and has a constant inside diameter corresponding to about a quarter of the wavelength of the microwave M.
The microwave M emitted from the microwave oscillaitor 121 is reflected by a plunger 123. A standing wave of the microwave M is formed by adjusting the position of the plunger 123. In this case, positioning is made so that a maximum field strength position M1 lies near an inner wall 100a of the discharge tube 100 and the gas contained in the discharge tube 100 is discharged at the portion of the maximum field strength position M1 to produce plasma.
However, the conventional discharge tube 100 has involved the following problem.
When the plasma is ignited with the energy at the maximum strength position Ml of the standing wave, the microwave transmission characteristic of the discharge tube 100 changes due to absorption of a radiant heat from the plasma and of the microwave M and, as indicated with a solid line and a broken line in FIG. 10, the maximum field strength position M1 of the microwave M shifts to the right or left from its initial position, resulting in the plasma blinking and being not stable. Further, the plunger 123, which is metallic, undergoes a thermal expansion and the distance between the plunger 123 and the discharge tube 100 changes. This sometimes results in that the plasma once produced vanishes suddenly after several minutes.
The conventional cylindrical discharge tube cannot cope with such a change in the maximum field strength position M1 of the microwave M which is produced in the initial stage of plasma formation, and the plasma blinks repeatedly. Thus, a long time is required until the plasma becomes stable and there occurs a phenomenon such that even if the plasma once becomes stable, it later vanishes. This causes a decrease in the number of workpieces treated per unit time, i.e., throughput, and a lowering of yield. Therefore, it has so far been keenly desired to remedy this point.
The present invention improves the above prior art and it is an object of the invention to provide a discharge tube for a local etching apparatus capable of producing a stable plasma in a short time in response to a variation of a standing wave of the microwave M which occurs at the beginning of or after plasma formation, capable of maintaining the stability of the plasma and thereby capable of improving the throughput and yield of workpieces, as well as a local etching apparatus using the discharge tube.
The discharge tube according to the present invention is for a local etching apparatus wherein a microwave emitted from a microwave oscillator into a waveguide is reflected by a plunger to produce a standing wave of the microwave, the discharge tube is positioned within the standing wave, allowing gas contained in the discharge tube to be converted to plasma, and then an active species contained in the plasma is jetted to a workpiece from an orifice of the discharge tube to etch the workpiece locally. A discharge portion of the discharge tube positioned within the waveguide is formed in a tapered shape which is divergent toward the orifice side of the discharge tube.
In this construction, the discharge tube is inserted through the waveguide and its discharge portion is positioned within the waveguide. Thereafter, a predetermined gas is fed into the discharge tube and the plunger is adjusted so that the portion corresponding to the maximum field strength of the standing wave is position near the inner wall of the discharge tube. In this state, a microwave is emitted from the microwave oscillator into the waveguide, where by a standing wave of the microwave is generated. In the case where the portion corresponding to the maximum field strength position of the standing wave is positioned near the inner wall of the discharge tube, the gases discharge into plasma. As a result, an active species contained in the plasma is jetted from the orifice of the discharge tube to the surface of the workpiece, whereby the workpiece is etched locally.
Even if the maximum field strength position of, the standing wave deviates from an initial set position, part of the maximum field strength position always lies near the inner wall of the discharge tube because in the discharge tube of the invention the discharge portion positioned in the waveguide is tapered so as to expand toward the orifice side. Thus, there is no fear of plasma blinking suddenly after ignition or vanishing after becoming stable. Not only a stable plasma can be obtained in a short time, but also the plasma can be kept stable.
A taper angle xcex8 between the wall of the discharge portion of the discharge tube and the axis of the same tube may be set in the range of 1xc2x0 to 10xc2x0. The material of the discharge tube may be any of quartz, aluminum oxide, and aluminum nitride.
The local etching apparatus according to the present invention comprises a microwave oscillator for generating a microwave, a waveguide for propagation therethrough of the microwave generated from the microwave oscillator, a plunger for reflecting the microwave propagated through the waveguide to generate a standing wave and for positioning a portion corresponding to a maximum field strength position of the standing wave at a predetermined position, a tapered discharge tube inserted through the waveguide and having a discharge portion positioned within the waveguide, the discharge portion being tapered so as to be divergent toward the orifice side from which the active species is jetted, and a workpiece scanning means which causes the orifice of the discharge tube to scan relatively on the surface of the workpiece opposed to the orifice.
The local etching apparatus may be further provided with cooling means for cooling the discharge tube by passing a cooling gas from a small-diameter side toward a large-diameter side of the tapered portion on an outer or inner surface of the discharge tube. The cooling gas is air or nitrogen and is flowed at a flow rate in the range from 30 to 90 xcex/m.
The local etching apparatus may be further provided with etching area limiting means which forms a gaseous atmosphere held at a predetermined pressure so as to surround the active species jetted from the orifice of the discharge tube toward the workpiece to prevent diffusion of the active species and which thereby limit the workpiece etching area by the active species to a smaller area than the area of the workpiece.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.